Laser printers have photosensitive drums that are selectively illuminated by light emitting diode arrays in response to an image to be printed. The exposure of the photosensitive drum generates a latent electrostatic image on its surface. This electrostatic image is developed by then exposing the photosensitive drum to toner which adheres to the charged portions of the drum. The toner on the photosensitive drum is then transferred to paper where it is fused by a fuser unit's application of heat. Usually, the fuser unit comprises a pair of opposed heated rollers between which the paper is fed. To fuse the toner to the paper, the fuser unit must be maintained at a temperature of approximately 150.degree. C. to 200.degree. C. A heater performs this function.
In conventional laser printers, the heater is controlled by the central processing unit of the laser printer. A typical example of a prior art system for controlling the temperature of a fuser unit of a laser printer is disclosed in U.S. Pat. No. 4,650,347 for Temperature Detector to Yutaka Shigemura, et al. As described in Shigemura, et al., a central processing means controls a three level analog-to-digital converter (shown in Shigemura, et al., as elements 534, 536, 540, 542) which generates digital signals indicative of a temperature of heating rollers. These digital signals are then provided to a controller which operates the heating element. In other conventional laser printers, a temperature sensor first detects the temperature of the fuser unit, an analog-to-digital converter then converts the detected temperature to a digital signal. The central processing unit receives this digital signal from latches controlled by address decoders and then determines whether or not the heater must be driven to maintain the fuser unit at the desired temperature.
Conventional laser printers in which the heater is controlled by the central processing unit have a number of drawbacks. First, the components necessary for converting the output of the temperature sensor into digital signals include the analog-to-digital converter, latches, and decoders. These additional components are costly which tends to increase the total cost of the laser printer. Moreover, since the central processing unit must constantly monitor the temperature of the fuser unit and control the driving of the heater, machine cycles of the central processing unit are required to perform these essentially mundane tasks. This factor is especially troublesome in the temperature detector of Shigemura, et al. where the central processing means must actually control the digital conversion operation of the analog-to-digital converter. Therefore, conventional laser printers must have either a faster central processing unit to control both the heater and the sensor in addition to the other functions of the laser printer or the overall speed of the laser printer must be slower.